An aircraft often has fuel tanks located in internal cavities in the aircraft's wings. The empty space in the fuel tanks that is not filled with fuel typically is a mixture of air and fuel vapor, and is referred to as ullage. To protect against fuel tank explosions, potentially combustible mixture of fuel vapor and air in the ullage space is replaced or diluted with an inert gas, such as nitrogen enriched air (NEA), to reduce the oxygen content in the ullage.
The fuel tanks generally are vented to the atmosphere to accommodate pressure changes with altitude during flight, thereby reducing the required strength, and thus weight, of the fuel tanks. While climbing, for example, the ambient pressure decreases as altitude increases. As a result, ullage gas in the fuel tank may be vented overboard to maintain pressure equilibrium or a specified pressure differential between the tank and the outside environment. The venting system typically vents the fuel tanks through a surge tank (an overflow tank that prevents unwanted ejection of fuel from the aircraft) located near a tip of the wing. The surge tank is provided with means for venting to atmosphere, and typically also includes means for draining fuel from the surge tank back to the fuel tanks.
Likewise, as an aircraft descends, the ambient pressure increases as the altitude decreases. Consequently, there may be a large inrush of ambient air into the ullage space during the descent. This is particularly true for an airplane with an open fuel tank vent, which is common on most commercial aircraft. Atmospheric air generally has about 21% oxygen content, so introduction of atmospheric air into the ullage space dilutes or spoils the inertness of the ullage. The inrush of atmospheric air can quickly raise the oxygen concentration in the ullage, which increases the combustion risk.
Additionally, frequently the pressure on the outlet of the venting system on one side of the aircraft will differ from the pressure on the other side of the aircraft. In a system with cross-venting, this may create a flow of gas that is large in magnitude through the venting system. If the center tank ullage is within this flow path, it must be assumed that this high magnitude of ambient air has polluted the air within the center tank and increased the oxygen concentration in the center tank ullage.
One way to address some of these problems is to provide a mixing chamber with a partition to prevent or minimize continuous flow from one side of the aircraft to the other and intermingling of air in the venting system through the ullage of the center fuel tank. U.S. Pat. No. 7,621,483 discloses such a mixing chamber.